Industrial emissions to the atmosphere, such as those from powerplants that combust fossil fuels to generate electricity, are subject to governmental regulation that is enforced by the United States Environmental Protection Agency (EPA).
Pursuant to statutory authority, the EPA has promulgated regulations that are embodied in portions of the Code of Federal Regulations (CFR). Included in portions of 40 CFR are regulations pertaining to measurement of volumetric flow rate of stack gas streams. While the regulations specify acceptable methods of measurement and types of probes that are introduced into stacks through test ports in the stack walls for obtaining those measurements, they leave it to industry to design and develop equipment for use with the probes that will enable the probes to be positioned within a stack for obtaining volumetric flow rate measurements in accordance with regulations.
The availability of portable electronic data recording equipment enables stack measurement data to be efficiently recorded on-site in electronic form and then later processed into proper reporting format for demonstrating regulatory compliance. The ability to automate a method for positioning a probe within a stack while electronically recording gas stream data is obviously desirable for increasing the efficiency and accuracy with which a test is performed.
Accordingly, it has been proposed to employ a motorized mechanism for positioning a probe within a stack as shown and described in various patents and publications, such as U.S. Pat. No. 5,440,217.
EPA regulations specify several test methods (Methods 2, 2F, and 2G) using certain specified probes. For performing Methods 2 or 2G, an S-type (“two dimensional”) probe is specified. For performing Method 2F, a prism head (“three dimensional”) probe is specified. The probe must translate in a direction that is transverse to the direction of the gas stream that is passing upward through the stack and it must also be capable of turning about the axis of translation. Such turning is referred to as yaw nulling.
The extent to which the probe needs to be advanced depends on the stack diameter. The larger the stack diameter, the greater the distance that the probe needs to be advanced. In very large diameter stacks, multiple test ports are provided at locations around the stack to allow a probe whose range of translation cannot span the full diameter to be placed at those locations and used for testing.
Because the extended probe acts in the manner of a cantilever on whatever structure is supporting it, and because the probe must be able to withstand hostile stack environments, the typical probe will have sufficient mass that will cause the probe to droop to some extent when maximally extended. The EPA test methods specify a maximum allowable droop of 5°.
Droop can be minimized by increasing probe stiffness, but increased stiffness is apt to require that probe mass and dimensions be increased, and when that is done, the construction of the mechanism that translates and turns the probe while at the same time supporting the cantilevered weight of the probe needs to be much more substantial, not only from the standpoint of structure but also from the standpoint of more powerful prime movers that are used to translate and turn the probe.
The device shown in U.S. Pat. No. 5,440,217 comprises two arrays of roller wheels that are spaced apart along the length of the probe and that bear against the outside of the cylindrical probe wall. Three roller wheels are journaled in roller assemblies that are arranged approximately equiangular about the probe wall in a first array and are forced against the probe wall by spring washers. In the other array, there are two roller assemblies like those of the first array, while the third roller wheel is a drive roller wheel that is coupled to a motor so that by virtue of friction between that roller wheel and the probe wall, rotation in one direction by the motor advances the probe and rotation in the opposite direction retracts the probe. The motor is a stepper motor that operates in increments.
The roller assemblies containing the non-driven roller wheels mount on a cylindrical housing within which the probe translates, with the probe increasingly protruding from that housing as the probe increasingly advances, and decreasingly protruding as the probe increasingly retracts. An alternative drive for probe translation is a chain drive as shown in U.S. Pat. No. 5,394,759.
Turning of the probe about the probe axis is accomplished by a second motor, also a stepper motor, that acts on the cylindrical housing containing the roller wheels that engage the probe wall. A timing belt is trained around the outside of the cylindrical housing wall and presumably a shaft or sheave of the second motor so that when the second motor operates, it turns the probe by turning the cylindrical housing within which the probe translates. The second motor is housed at one end of an outer housing assembly, whose other end is fit and secured to a mounting ring on the stack at the stack test port. The outer housing assembly surrounds the cylindrical housing containing the roller wheels that engage the probe, for at least some of the length of the cylindrical housing.
Encoders track translation and turning of the probe. The motors are controlled by a computer that calculates the points at which the probe is to be positioned for testing and will output signals to the stepper motors for translating the probe to the desired test point and turning the probe to the desired angular orientation.
Analysis of the devices shown in the referenced US Patents discloses that a more robust automated probe would be desirable. Some aspects of the patented probes that may compromise robustness include: translational accuracy of the probe; the use of spring washers in one version to force the roller wheels against the probe wall in the apparent interest of providing adequate cantilever support, but at the same time creating additional stresses that must be accommodated by mechanical strengthening, which typically means added mass, and motor size large enough not only to move the probe but in doing so to also overcome the opposing force components of the spring washers; and the use of two housings, one within another, adding complexity and weight.